Composition for thick oxide superconductor film and method of producing oxide superconductor in form of thick film tape using the same

ABSTRACT

To provide a composition for a thick oxide superconducting film containing a copper salt of a branched saturated aliphatic carboxylic acid having 6 or more carbon atoms and/or a copper salt of an alicyclic carboxylic acid having 6 or more carbon atoms, which is suitable for producing thick copper based oxide superconducting films by the MOD process and which can be subjected to film formation with uniformity at a high speed, and an oxide superconductor in the form of a thick film tape which is subjected to film formation with uniformity at a high speed using the subject composition for a thick oxide superconducting film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application (35 USC 371) ofPCT/JP2003/014178 and claims priority of Japanese Application No.2002-325812 filed Nov. 8, 2002.

TECHNICAL FIELD

The present invention relates to a composition for a thick oxidesuperconducting film by the metal organic deposition process(hereinafter sometimes referred to as “MOD process”) for forming aceramic on a substrate by heating and/or baking a precursor and to anoxide superconductor in the form of a thick film tape using the subjectcomposition, and in particular, it relates to a composition for a thickoxide superconducting film which can be subjected to film formation withuniformity at a high speed and to an oxide superconductor in the form ofa thick film tape which is subjected to film formation with uniformityat a high speed.

BACKGROUND ART

In oxide superconductors, since the critical temperature (T_(c)) thereofexceeds the temperature of liquid nitrogen, their applications to wires,devices, etc. are expected, and various studies have been vigorouslymade.

In particular, in order to apply oxide superconductors to wires, it isnecessary to produce oxide superconductor shaving a high criticalcurrent density (J_(c)) and a long length. On the other hand, in orderto obtain long oxide superconductors in the form of a tape, it isnecessary to form an oxide superconductor on a metal tape from theviewpoints of strength and flexibility.

Also, since oxide superconductors have anisotropy in thecrystallography, in order to improve the J_(c), it is necessary toestablish a film formation process for epitaxially growing an oxidesuperconductor on an aligned substrate.

As processes for producing a tape-formed RE based oxide superconductor,i.e., RE_(1+x)Ba_(2−x)Cu₃O_(y) (wherein RE represents at least oneelement selected from the group consisting of Y, Nd, Sm, Gd, Eu, Yb, Prand Ho; x represents the number of 0≦x≦0.4; and y represents the numberof 6.5≦y≦7.0, hereinafter the same) based oxide superconductor, there isknown the MOD process.

This MOD process (Metal Organic Deposition process) is to thermallydecompose organic acid metal salts and is a process for coating asolution having a metal component-containing organic compound uniformlydissolved therein on a substrate and then heating it for thermaldecomposition, thereby forming a thick film on the substrate. The MODprocess has such an advantage that since not only a high J_(c) isobtained in a non-vacuum process, but also high-speed film formation canbe achieved at low costs, this process is suitable for the production ofoxide superconducting wires in the form of a tape.

In the MOD process, when a starting material containing metal organicacid salts is thermally decomposed, a carbonate of an alkaline earthmetal (such as Ba) is usually formed. In the formation of an oxidesuperconductor by a solid phase reaction via such a carbonate, ahigh-temperature heat treatment at 800° C. or higher is necessary. Also,a lowering of the J_(c) caused due to film thickening becomes a seriousproblem.

Against the foregoing problems, in recent years, there have beenvigorously carried out processes for forming an RE (123) superconductor(RE:Ba:Cu=1:2:3, hereinafter the same) using fluorine-containing organicacid salts (for example, a TFA salt: trifluoroacetic acid salt) as thestarting material by a heat treatment in a water vapor atmosphere whilecontrolling a water vapor partial pressure (for example, see PatentDocument 1). According to the process using this TFA salt as thestarting material, it is possible to epitaxially grow the RE (123)superconductor from a substrate by reaction of water vapor and afluorine-containing amorphous precursor. Concretely, after coating thestarting material solution on the substrate, the coated substrate isthermally treated for crystallization at 750° C. via a calcination stepfor thermally treating it at not higher than 400° C. for the purpose ofdecomposing organic components, thereby forming a superconducting film.As characteristics of the heat treatment in the present process, thereis enumerated the generation of an HF gas by reaction of fluorine andwater vapor in the film. In particular, in the calcination step, a largeamount of HF gas is generated due to the decomposition of coppertrifluoroacetate into CuO. For this reason, in order to preventcracking, etc. in the film due to abrupt decomposition reaction fromoccurring, it is necessary to control the heating rate in thecalcination step at not more than 1° C./min. Also, taking intoconsideration the solubility of the TFA salt, highly volatile methanolmust be chosen as a solvent. Accordingly, a change of the startingmaterial solution with time is vigorous even at room temperature, andits viscosity becomes higher with an increase of the concentration.

In the light of the above, since the trifluoroacetic acid metal salt issoluble in only a low boiling solvent such as methanol, the solvent isvaporized during the coating work of the starting material solution onthe substrate so that uniform coating cannot be achieved. Thus, theuniformity of the resulting thick oxide superconducting film was low.Also, since it takes a long period of time for the calcination stepprior to the actual baking, the high-speed film formation was difficult.

Accordingly, an object of the present invention is to provide acomposition for a thick oxide superconducting film capable of beingsubjected to film formation with uniformity at a high speed, which issuitable for producing thick copper based oxide superconducting films bythe MOD process, and an oxide superconductor in the form of a thick filmtape which is subjected to film formation with uniformity at a highspeed using the subject composition for a thick oxide superconductingfilm.

[Patent Document 1] U.S. Pat. No. 5,231,074

DISCLOSURE OF THE INVENTION

The present inventors have found that by using a copper salt of abranched saturated aliphatic carboxylic acid having 6 or more carbonatoms and/or a copper salt of an alicyclic carboxylic acid having 6 ormore carbon atoms in place of the conventionally used coppertrifluoroacetate, it is possible to set up the heating rate in thecalcination step at 2° C./min or more.

The invention has been made based on the foregoing finding and is toprovide a composition for a thick oxide superconducting film, containinga copper salt of a branched saturated aliphatic carboxylic acid having 6or more carbon atoms and/or a copper salt of an alicyclic carboxylicacid having 6 or more carbon atoms.

Also, the invention is to provide the foregoing composition for a thickoxide superconducting film containing an organic solvent having aboiling point of 80° C. or higher as a solvent. By using the organicsolvent having a boiling point of 80° C. or higher, which is low invapor pressure at room temperature, as a solvent, it is possible toprepare an oxide superconductor in the form of a thick film tape havinga uniform thickness, which is small in a change of its concentrationeven when it is allowed to stand for a long period of time in the air.

Also, the invention is to provide an oxide superconductor in the form ofa thick film tape, subjecting an oxide superconducting precursor, whichis obtained by coating the foregoing composition for a thick oxidesuperconducting film on a substrate and then subjecting it to a heattreatment for calcination, to a heat treatment for crystallization,thereby forming a thick oxide superconducting film on the foregoingsubstrate.

The invention will be described below in detail.

The composition for a thick oxide superconducting film of the inventioncan be well soluble in an organic solvent having a relatively highboiling point even without using a low boiling solvent which hashitherto been used in compositions of such a type and contains a coppersalt of a branched saturated aliphatic carboxylic acid having 6 or morecarbon atoms and/or a copper salt of an alicyclic carboxylic acid having6 or more carbon atoms as an essential component.

Not only such a copper salt per se is readily soluble in an organicsolvent having a relatively high boiling point, but also it has such aneffect that it is able to make other components than the copper saltsoluble in the organic solvent having a relatively high boiling point.

The branched saturated aliphatic carboxylic acid and/or the alicycliccarboxylic acid which constitutes the copper salt has 6 or more carbonatoms, and preferably 8 or more carbon atoms. When the number of carbonatoms is too small, it is difficult to obtain the effect for makingother components than the copper salt soluble in the high boilingsolvent.

The upper limit of the number of carbon atoms of such a carboxylic acidis not particularly limited. However, when the number of carbon atoms isextremely large, the copper salt per se becomes hardly soluble in thesolvent, and therefore, the number of carbon atoms is preferably notmore than 16, and more preferably not more than 12.

The copper salt of a branched saturated aliphatic carboxylic acid having6 or more carbon atoms and/or the copper salt of an alicyclic carboxylicacid having 6 or more carbon atoms is most preferably at least kindselected from the group consisting of copper neodecanoate, copperisononanoate, copper 2-ethylhexanoate, and copper naphthenate.

The composition for a thick oxide superconducting film of the inventionis a copper based composition for a thick oxide superconducting filmcontaining the foregoing specific copper salt as a copper precursorcompound and can arbitrarily contain conventionally known compounds asprecursor compounds other than copper. The content of the foregoingcopper salt in the composition for a thick oxide superconducting film ofthe invention is preferably from 10 to 60% by weight, and especiallypreferably from 20 to 45% by weight.

For example, in the case of a Y—Ba—Cu based composition for a thickoxide superconducting film, the composition for a thick oxidesuperconducting film of the invention can contain an yttrium precursorcompound and a barium precursor compound along with the foregoing coppersalt.

The foregoing yttrium precursor compound is not particularly limited,and known yttrium precursor compounds can be used. However, yttriumtrifluoroacetate is preferable from the viewpoint of film formingproperties.

Also, as the foregoing yttrium precursor compound, yttrium salts of abranched saturated aliphatic carboxylic acid having 6 or more carbonatoms and yttrium salts of an alicyclic carboxylic acid having 6 or morecarbon atoms are preferable. The branched saturated aliphatic carboxylicacid having 6 or more carbon atoms and the alicyclic carboxylic acidhaving 6 or more carbon atoms, each of which constitutes such an yttriumsalt, are the same as those constituting the foregoing copper salt.

The foregoing barium precursor compound is not particularly limited, andknown barium precursor compounds can be used. However, bariumtrifluoroacetate is preferable from the viewpoint of film formingproperties.

In the composition for a thick oxide superconducting film of theinvention, the content of the foregoing copper salt as the yttriumprecursor compound, the barium precursor compound, and the copperprecursor compound is not particularly limited, and the ratio of therespective metal components may be the same as in conventionally knowncompositions for a thick oxide superconducting film. For example, anyttrium/barium/copper molar ratio is preferably 1/(1.8 to 2.2)/(2.4 to3.6).

It is preferable that the composition for a thick oxide superconductingfilm of the invention contains an organic solvent which dissolves theseprecursor compounds therein as the solvent. The subject organic solventis not particularly limited so far as it dissolves these precursorcompounds therein. Examples thereof include 2-octanone, 3-pentanone,3-heptanone, 4-heptanone, 4-methyl-2-pentanone,2,6-dimethyl-4-heptanone, and 1-pentanol. Of these organic solvents,organic solvents having a boiling point of 80° C. or higher arepreferable; and 2-octanone is especially preferable.

When a low boiling solvent is used, in producing an oxide superconductorin the form of a thick film tape as described later using thecomposition for a thick oxide superconducting film, the low boilingsolvent is vaporized in a step for coating the composition for a thickoxide superconducting film on a substrate, whereby the concentration ofthe composition for a thick oxide superconducting film is changed(concentrated), and therefore, it likely becomes difficult to achieveuniform coating. However, by using an organic solvent having a boilingpoint of 80° C. or higher as the solvent, it becomes possible to achieveuniform coating.

The content of the foregoing organic solvent in the composition for athick oxide superconducting film of the invention is preferably from 25to 80% by weight, and especially preferably from 35 to 60% by weight.

The composition for a thick oxide superconducting film of the inventionmay further contain arbitrary components such as a thickener, astabilizer, a surfactant, and a dispersant as the need arises.Incidentally, it is preferable that the content of these arbitrarycomponents in the composition for a thick oxide superconducting film ofthe invention be not more than 10% by weight.

Next, the oxide superconductor in the form of a thick film tape of theinvention, which is obtained by the composition for a thick oxidesuperconducting film of the invention, will be described below alongwith its production embodiment.

In the production of the oxide superconductor in the form of a thickfilm tape of the invention, first of all, the foregoing composition fora thick oxide superconducting film of the invention is coated on asubstrate.

The foregoing substrate is not particularly limited but can be properlychosen among known substrates which can form a thick oxidesuperconductive film by the metal organic process. Examples thereofinclude metallic tapes and metallic tapes provided with an interlayer.

As the foregoing substrate, any of singlecrystalline substrates andpolycrystalline substrates can be used. Examples of the foregoingsinglecrystalline substrates include a LaAlO₃ (100) singlecrystallinesubstrate (LAO singlecrystalline substrate). On the other hand, examplesof the polycrystalline substrates include textured Ni substrates andcomposite substrates using an IBAD (Ion Beam Assisted Deposition)process.

The method of coating the composition for a thick oxide superconductingfilm of the invention on the foregoing substrate is not particularlylimited but can be properly chosen among known coating methods whichhave hitherto been employed in forming a thick oxide superconductingfilm by the metal organic process. Examples thereof include adip-coating process and a brush coating process.

Next, the composition for a thick oxide superconducting film of theinvention having been coated on a substrate is subjected to a heattreatment for calcination to obtain an oxide superconducting precursor.Preferred conditions in this heat treatment for calcination will bedescribed below in detail.

In the foregoing heat treatment for calcination, it is possible toincrease a heating rate. In this way, it is possible to subject theoxide superconductor in the form of a thick film tape of the inventionto film formation at a high speed. The subject heating rate ispreferably 2° C./min or more, and especially preferably 3° C./min ormore. The upper limit thereof is not particularly limited so far as thefilm formation is possible but is usually approximately 5° C./min.

Also, in the foregoing heat treatment for calcination, it is possible toincrease the product of the traveling speed of the substrate and thetemperature gradient. In this way, it is possible to subject the oxidesuperconductor in the form of a thick film tape of the invention to filmformation in a small electric furnace. The product of the travelingspeed of the substrate and the temperature gradient is preferably 2°C./min or more, and especially preferably 3° C./min or more. The upperlimit thereof is not particularly limited so far as the film formationis possible but is usually approximately 5° C./min.

Also, it is preferable that the foregoing heat treatment for calcinationbe carried out at a temperature of 250° C. or higher, and especially inthe range of from 300 to 500° C. in the atmosphere where the water vaporpartial pressure is not more than 2.1% by volume, and especially from0.1 to 1.0% by volume.

Next, by subjecting the oxide superconducting precursor obtained byundergoing a heat treatment for calcination to a heat treatment forcrystallization, there is obtained the oxide superconductor in the formof a thick film tape of the invention in which the thick oxidesuperconducting film is formed on the substrate. This heat treatment forcrystallization may be carried out by the customary manner. For example,it is preferable that the heat treatment for crystallization be carriedout at a heat treatment temperature in the range of from 725 to 775° C.in the atmosphere where the water vapor partial pressure is from 2.1 to20% by volume.

The thus obtained oxide superconductor in the form of a thick film tapeof the invention, especially the oxide superconductor in the form of athick film tape of the invention containing an organic solvent having aboiling point of 80° C. or higher, has a thick oxide superconductingfilm having a uniform thickness. A difference between the maximumthickness portion and the minimum thickness portion in the foregoingthick oxide superconducting film is preferably not more than 1 μm, andespecially preferably not more than 0.5 μm.

Also, by using the organic solvent having a boiling point of 80° C. orhigher, the oxide superconductor in the form of a thick film tape of theinvention has a uniform thickness, and by subjecting it to the foregoingheat treatment for crystallization, it is possible to control an amountof change in the critical current density at a small level. The amountof change in the critical current density is preferably within the rangeof ±0.5 MA/cm².

Also, in the oxide superconductor in the form of a thick film tape ofthe invention, the thick oxide superconducting film to be formed on thesubstrate is preferably comprised of RE_(1+x)Ba_(2−x)Cu₃O_(y).

Examples of applications of the oxide superconductor in the form of athick film tape of the invention include wires, devices, or electricinstruments such as power cables, transformers and fault currentlimiters.

The invention will be further described below with reference to theExamples, but it should not be construed that the invention is limitedthereto.

EXAMPLE 1

(Production of Yttrium Trifluoroacetate Trihydrate)

A one-liter reaction flask was charged with 75 g of water and 132 g oftrifluoroacetic acid, to which was then added 39.66 g of diyttriumtrioxide while stirring. After completion of the generation of heat, themixture was refluxed for 2 hours, and 10 g of methanol was then added toa solution obtained by filtering off insoluble matters. The temperaturewas raised to 70° C., and the system was gradually evacuated, wherebythe reaction mixture was concentrated and evaporated to dryness. Afterthe evaporation to dryness, the system was returned to atmosphericpressure while introducing nitrogen, and the resulting solid was takenout under a nitrogen gas stream to obtain 315 g (percent yield: 92.9%)of a crystal. The obtained crystal was subjected to IR analysis, yttriumcontent analysis (thermally decomposed at 500° C. and weighed as atrifluoride), and differential thermal analysis in air. As a result ofthese analyses, the resulting crystal was identified to be yttriumtrifluoroacetate trihydrate as a desired product. The analysis resultsare shown below.

<Analysis Results>

-   -   IR analysis: Absorption peaks (cm⁻¹)

3664, 3424, 1716, 1666, 1625, 1484, 1467, 1457, 1218, 1151, 865, 842,800, 759, 732, 678

-   -   Yttrium content analysis: 18.5% by weight (theoretical value:

18.45% by weight)

-   -   Differential thermal analysis: (at 30° C., 10° C./min, 600° C.)

30° C. (−0.0%: trihydrate), 117° C. (−7.5%: monohydrate), 155° C.(−11.4%: anhydride), 310° C. (−73.2%: yttrium trifluoride)

(Production of Barium Trifluoroacetate Monohydrate)

A one-liter reaction flask was charged with 260 g of water and 315 g ofbarium hydroxide octahydrate, to which was then dropped 240 g oftrifluoroacetic acid at not higher than 40° C. while stirring. Afterreacting at 25° C. for 2 hours, 20 g of methanol was added to a solutionobtained by filtering off insoluble matters. The temperature was raisedto 70° C., and the system was gradually evacuated, whereby the reactionmixture was concentrated and evaporated to dryness. After theevaporation to dryness, the system was returned to atmospheric pressurewhile introducing nitrogen, and the resulting solid was taken out undera nitrogen gas stream to obtain 367 g (percent yield: 96.2%) of acrystal. The obtained crystal was subjected to IR analysis, bariumcontent analysis (sulfuric acid hydrothermal decomposition method), andcomposition analysis of a residue after heating at 400° C. in air (X-rayanalysis). As a result of these analyses, the resulting crystal wasidentified to be barium trifluoroacetate monohydrate as a desiredproduct. The analysis results are shown below.

<Analysis Results>

-   -   IR analysis: Absorption peaks (cm⁻¹)

3667, 3432, 1673, 1454, 1213, 1145, 850, 802, 759, 727, 678

-   -   Barium content analysis: 36.3% by weight (theoretical value:

36.01% by weight)

-   -   Composition analysis after heating at 400° C.: BaF₂        (Production of Copper Neodecanoate)

A three-liter reaction flask was charged with 293 g of copper hydroxideand 938 g of isopropanol, to which was then added 1,137 g of neodecanoicacid while stirring. After heating for reflux for 2 hours, the systemwas gradually evacuated to distill off the isopropanol and water. 1,294g of toluene was added to that solution, and insoluble matters werefiltered off. The resulting solution was again gradually evacuated todistill off the toluene and water, followed by concentration to obtain aviscous liquid. After the concentration, the system was returned toatmospheric pressure while introducing nitrogen, and the resultingviscous liquid was taken out under a nitrogen gas stream to obtain 1,028g (percent yield: 85.2%) of a liquid. The obtained liquid was subjectedto IR analysis, copper content analysis (thermally decomposed at 800° C.and weighed as copper oxide), and composition analysis of a residueafter heating (X-ray analysis). As a result of these analyses, theresulting liquid was identified to be copper neodecanoate as a desiredproduct. The analysis results are shown below.

<Analysis Results>

-   -   IR analysis: Absorption peaks (cm⁻¹)

3617, 3448, 3153, 2964, 2873, 1697, 1600, 1535, 1465, 1411, 1376, 1299,1234, 1174, 1004, 786, 653, 484

-   -   Copper content analysis: 15.35% by weight (theoretical value:

15.66% by weight)

-   -   Composition analysis after heating at 800° C.: CuO        (Preparation of Composition for Thick Oxide Superconducting        Film)

1.7 g of the obtained yttrium trifluoroacetate trihydrate, 2.9 g of theobtained barium trifluoroacetate monohydrate, and 4.9 g of the obtainedcopper neodecanoate (in terms of a metal atom molar ratio of Y/Ba/Cu of1/2/3.05) were mixed, to which was then added and stirred 9.2 g of2-octanone (at 28° C.). As a result, the mixture was uniformlydissolved. This precursor solution was designated as a composition 1.

(Film Formation of Oxide Superconductor in Form of Thick Film Tape)

An IBAD composite substrate comprising Hastelloy, Gd₂Zr₂O₇ and CeO₂ wasused as a substrate. This IBAD composite substrate was prepared by filmformation of a first interlayer of Gd₂Zr₂O₇ in a thickness of 1 μm on aHastelloy tape having a length of 10 m, a width of 10 mm and a thicknessof 0.1 mm at room temperature using the IBAD process and film formationof a second interlayer of CeO₂ in a thickness of 0.5 μm thereon using asputtering process.

After coating the foregoing composition 1 on this IBAD compositesubstrate by a dip coating process, the coated substrate was calcined to500° C. at a heating rate of 0.2 to 5° C./min (see Tables 1 to 3) in theatmosphere having a water vapor partial pressure of 2.1% by volume andan oxygen partial pressure of 97.9% by volume and then subjected tofurnace cooling in the atmosphere having an oxygen partial pressure of100%. Subsequently, the coating and calcination were repeated in thesame manner (with respect to the number of times, see Tables 1 to 3),thereby obtaining Y—Ba—Cu precursors. Thereafter, each of the subjectprecursors was heated at a heating rate of 25° C./min in the atmospherehaving a water vapor partial pressure of 6.3% by volume and an oxygenpartial pressure of 93.7% by volume and baked at a substrate temperatureof 750° C. for 1 to 3 hours. Thereafter, the atmosphere within thefurnace was switched into a dry gas, and the baked precursor was keptfor 10 minutes, followed by furnace cooling to obtain an oxidesuperconductor in the form of a thick film tape. The thickness resultingfrom one-time coating was 0.3 μm.

Silver was subjected to vapor deposition on the thick oxidesuperconducting film on the resulting oxide super-conductor in the formof a thick film tape to form an electrode, which was then thermallytreated in the oxygen atmosphere at 450° C. for one hour.

A 10 cm-long sample was prepared from each of the oxide superconductorsin the form of a thick film tape (Examples 1-1 and 1-2) obtained at aheating rate in the calcination of 0.2° C./min and at the number oftimes of coating and calcination of 1 or 3 and then measured for J_(c)and I_(c) (critical current) in liquid nitrogen. The measurement resultsare shown in Table 1.

A 10 cm-long sample was prepared from the oxide superconductor in theform of a thick film tape (Example 1-3) obtained at a heating rate inthe calcination of 5° C./min and at the number of times of coating andcalcination of 3 and then measured for J_(c) and I_(c) in liquidnitrogen. The measurement results are shown in Table 2 along with themeasurement results of the oxide superconductor in the form of a thickfilm tape obtained in Example 1-2.

Further, with respect to the oxide superconductor in the form of a thickfilm tape obtained in Example 1-2 (10 cm-long sample), the maximumthickness and minimum thickness and the maximum I_(c) and minimum I_(c)were measured, thereby examining the uniformity in the longitudinaldirection. The measurement results are shown in Table 3.

Comparative Example

TFA salts of respective metals were dissolved in methanol such that themolar ratio of Y/Ba/Cu became 1/2/3.05, and the concentration of thesolution was adjusted at 0.25 moles/liter as reduced into Y, therebypreparing a comparative composition. Oxide superconductors (ComparativeExamples 1-1 to 1-3) were prepared in the same manners as in Examples1-1 to 1-3, except for using the subject comparative composition as thestarting material solution in place of the composition 1, and thenmeasured for various characteristics. The measurement results are shownin Tables 1 to 3.

TABLE 1 Example Example Comparative Comparative 1-1 1-2 Example 1-1Example 1-2 Thickness (μm) 0.3 0.9 0.3 0.9 Number of times of 1 3 1 3coating Heating rate 0.2 0.2 0.2 0.2 (° C./min) J_(c) (MA/cm²) 5.2 3.55.3 3.7 I_(c) (A) 156 315 159 333

TABLE 2 Example Example Comparative Comparative 1-2 1-3 Example 1-2Example 1-3 Thickness (μm) 0.9 0.9 0.9 0.9 Number of times of 3 3 3 3coating Heating rate 0.2 5 0.2 5 (° C./min) J_(c) (MA/cm²) 3.5 3.6 3.70.2 I_(c) (A) 315 324 333 18

TABLE 3 Example 1-2 Comparative Example 1-2 Number of times of coating 33 Minimum thickness (μm) 0.81 0.83 Maximum thickness (μm) 0.99 2.0Minimum I_(c) (A) 280 25 Maximum I_(c) (A) 347 333

As is evident from the results shown in Table 1, in the oxidesuperconductors in the form of a thick film tape of the invention(Examples 1-1 and 1-2) as prepared using the composition for a thickoxide superconducting film of the invention, characteristicssubstantially the same as those in the oxide superconductors in the formof a thick film tape (Comparative Examples 1-1 and 1-2) as preparedusing the conventional composition for a thick oxide superconductingfilm were obtained. However, the composition for a thick oxidesuperconducting film of the invention (composition 1) could be uniformlycoated on the substrate without causing vaporization of the solventduring the coating step, whereas the conventional composition for athick oxide superconducting film (comparative composition) could not beuniformly coated while causing volatilization of the methanol during thecoating step.

Also, as is evident from the results shown in Table 2, in the oxidesuperconductor in the form of a thick film tape of the invention(Example 1-3), even in the case of the preparation of a fast heatingrate, J_(c) and I_(c) were not lowered, and the calcination could beachieved within a short period of time. In contrast, in the oxidesuperconductor in the form of a thick film tape as prepared at a fastheating rate using the conventional composition for a thick oxidesuperconducting film (Comparative Example 1-3), J_(c) and I_(c) wereremarkably lowered.

Moreover, it was confirmed from the results shown in Table 3 that in theoxide superconductor in the form of a thick film tape of the invention(Example 1-2), scatters of the thickness and I_(c) values were within±10%. In contrast, in the oxide superconductor in the form of a thickfilm tape as prepared using the conventional composition for a thickoxide superconducting film (Comparative Example 1-2), concentration ofthe starting material solution was noted due to volatilization of themethanol in the starting material solution during the coating step. Forthat reason, a scatter of the thickness of the oxide superconductor inthe form of a thick film tape was large as a wire, and in the thickportion, cracking was generated during the calcination step, and alowering of J_(c) was largely observed.

In the light of the above, it was noted that the composition for a thickoxide superconducting film of the invention using a high boiling solventis free from a change of the concentration with time, from which auniform wire can be prepared at a high speed.

EXAMPLE 2

(Production of Copper 2-ethylhexanoate)

A two-liter reaction flask was charged with 112 g of copper hydroxideand 688 g of ethanol, to which was then added 341 g of 2-ethylhexanoicacid while stirring. After heating for reflux at 80° C. for 4 hours, 120g of water was added, and the mixture was heated for reflux for anadditional one hour. After the reflux, the system was graduallyevacuated to distill off the ethanol and water, thereby obtaining asolid. 400 g of ethanol was added to the obtained solid, and insolublematters were filtered off. The resulting solution was again graduallyevacuated, whereby the reaction mixture was concentrated and evaporatedto dryness. After the evaporation to dryness, the system was returned toatmospheric pressure while introducing nitrogen, and the resulting solidwas taken out under a nitrogen gas stream to obtain 302 g (percentyield: 75.1%) of a crystal. The obtained crystal was subjected to IRanalysis, copper content analysis (thermally decomposed at 800° C. andweighed as copper oxide), and composition analysis of a residue afterheating (X-ray analysis). As a result of these analyses, the resultingcrystal was identified to be copper 2-ethylhexanoate as a desiredproduct. The analysis results are shown below.

<Analysis Results>

-   -   IR analysis: Absorption peaks (cm⁻¹)

3446, 2958, 2935, 2873, 1594, 1517, 1457, 1421, 1378, 1322, 1236, 1106,808, 761, 732, 692, 601, 457

-   -   Copper content analysis: 18.27% by weight (theoretical value:        18.16% by weight)    -   Composition analysis after heating at 800° C.: CuO        (Preparation of Composition for Thick Oxide Superconducting Film        and Film Formation of Oxide Superconductor in Form of Thick Film        Tape)

A precursor solution (composition 2) was obtained in the same manner asin Example 1, except for using 6.7 g of the obtained copper2-ethylhexanoate in place of 4.9 g of the copper neodecanoate.

An oxide superconductor in the form of a thick film tape was prepared inthe same manner as in Example 1, except for using the foregoingcomposition 2 in place of the composition 1, and then measured forvarious characteristics. As a result, there were obtained good resultsthe same as in Example 1.

EXAMPLE 3

(Production of Copper Isononanoate)

A 300-mL reaction flask was charged with 15 g of copper hydroxide, 100 gof water, and 80 g of isopropanol, to which was then added 50 g ofisononanoic acid while stirring. After stirring at 50° C. for one hour,the system was gradually evacuated to distill off the isopropanol andwater. 43 g of toluene was added to that solution, and the resultingsolution was again gradually evacuated to distill off the toluene andwater, followed by concentration to obtain a solid. 130 g of toluene wasagain added to the resulting solid, and a solution obtained by filteringoff insoluble matters was again gradually evacuated, whereby thereaction mixture was concentrated and evaporated to dryness. After theevaporation to dryness, the system was returned to atmospheric pressurewhile introducing nitrogen, and the resulting solid was taken out undera nitrogen gas stream to obtain 44 g (percent yield: 77.6%) of acrystal. The obtained crystal was subjected to IR analysis, coppercontent analysis (thermally decomposed at 800° C. and weighed as copperoxide), and composition analysis of a residue after heating (X-rayanalysis). As a result of these analyses, the resulting crystal wasidentified to be copper isononanoate as a desired product. The analysisresults are shown below.

<Analysis Results>

-   -   IR analysis: Absorption peaks (cm⁻¹)

3438, 2956, 2902, 2869, 1581, 1508, 1417, 1365, 1319, 1247, 759, 678,667

-   -   Copper content analysis: 15.92% by weight (theoretical value:        16.81% by weight)    -   Composition analysis after heating at 800° C.: CuO        (Preparation of Composition for Thick Oxide Superconducting Film        and Film Formation of Oxide Superconductor in Form of Thick Film        Tape)

A precursor solution (composition 3) was obtained in the same manner asin Example 1, except for using 8.0 g of the obtained copper isononanoatein place of 4.9 g of the copper neodecanoate.

An oxide superconductor in the form of a thick film tape was prepared inthe same manner as in Example 1, except for using the foregoingcomposition 3 in place of the composition 1, and then measured forvarious characteristics. As a result, there were obtained good resultsthe same as in Example 1.

EXAMPLE 4

(Production of Copper Naphthenate)

A three-liter reaction flask was charged with 195 g of copper hydroxideand 625 g of isopropanol, to which was then added 981 g of naphthenicacid while stirring. After heating for reflux for 2 hours, the systemwas gradually evacuated to distill off the isopropanol and water. 860 gof toluene was added to that solution, and insoluble matters werefiltered off. The resulting solution was again gradually evacuated todistill off the toluene and water, followed by concentration to obtain aviscous liquid. After the concentration, the system was returned toatmospheric pressure while introducing nitrogen, and the resultingviscous liquid was taken out under a nitrogen gas stream to obtain 884 g(percent yield: 83.6%) of a liquid. The obtained liquid was subjected toIR analysis, copper content analysis (thermally decomposed at 800° C.and weighed as copper oxide), and composition analysis of a residueafter heating (X-ray analysis). As a result of these analyses, theresulting crystal was identified to be copper naphthenate as a desiredproduct. The analysis results are shown below.

<Analysis Results>

-   -   IR analysis: Absorption peaks (cm⁻¹)

3436, 2925, 2867, 1704, 1589, 1540, 1456, 1417, 1376, 1305, 734, 686

-   -   Copper content analysis: 7.8% by weight    -   Composition analysis after heating at 800° C.: CuO        (Preparation of Composition for Thick Oxide Superconducting Film        and Film Formation of Oxide Superconductor in Form of Thick Film        Tape)

A precursor solution (composition 4) was obtained in the same manner asin Example 1, except for using 9.9 g of the obtained copper naphthenatein place of 4.9 g of the copper neodecanoate.

An oxide superconductor in the form of a thick film tape was prepared inthe same manner as in Example 1, except for using the foregoingcomposition 4 in place of the composition 1, and then measured forvarious characteristics. As a result, there were obtained good resultsthe same as in Example 1.

INDUSTRIAL APPLICABILITY

According to the invention, it is possible to provide a composition fora thick oxide superconducting film capable of being subjected to filmformation with uniformity at a high speed, which is suitable forproducing thick copper based oxide superconducting films by the MODprocess, and an oxide superconductor in the form of a thick film tapeusing the subject composition for a thick oxide superconducting film.

1. A composition for a thick oxide superconducting film, containing acopper salt of a branched saturated aliphatic carboxylic acid having 6or more carbon atoms or a copper salt of an alicyclic carboxylic acidhaving 6 or more carbon atoms, and further containing yttriumtrifluoroacetate and barium trifluoroacetate.
 2. The composition for athick oxide superconducting film according to claim 1, wherein thecopper salt of a branched saturated aliphatic carboxylic acid having 6or more carbon atoms or the copper salt of an alicyclic carboxylic acidhaving 6 or more carbon atoms is at least one kind selected from thegroup consisting of copper neodecanoate, copper isononanoate, copper2-ethylhexanoate, and copper naphthenate.
 3. The composition for a thickoxide superconducting film according to claim 2, containing an organicsolvent having a boiling point of 80° C. or higher as a solvent.
 4. Thecomposition for a thick oxide superconducting film according to claim 3,which is characterized in that the foregoing organic solvent is2-octanone.
 5. A composition for a thick oxide superconducting film,containing a copper salt of a branched saturated aliphatic carboxylicacid having 6 or more carbon atoms or a copper salt of an alicycliccarboxylic acid having 6 or more carbon atoms, and containing an yttriumsalt of a branched saturated aliphatic carboxylic acid having 6 or morecarbon atoms or an yttrium salt of an alicyclic carboxylic acid having 6or more carbon atoms, and further containing barium trifluoroacetate. 6.The composition for a thick oxide superconducting film according toclaim 5, wherein the copper salt of a branched saturated aliphaticcarboxylic acid having 6 or more carbon atoms or the copper salt of analicyclic carboxylic acid having 6 or more carbon atoms is at least onekind selected from the group consisting of copper neodecanoate, copperisononanoate, copper 2-ethylhexanoate, and copper naphthenate.
 7. Thecomposition for a thick oxide superconducting film according to claim 6,containing an organic solvent having a boiling point of 80° C. or higheras a solvent.
 8. The composition for a thick oxide superconducting filmaccording to claim 7, which is characterized in that the foregoingorganic solvent is 2-octanone.
 9. A method of producing an oxidesuperconductor in the form of a thick film tape comprising the steps of:coating a composition for a thick oxide superconducting film, containinga copper salt of a branched saturated aliphatic carboxylic acid having 6or more carbon atoms or a copper salt of an alicyclic carboxylic acidhaving 6 or more carbon atoms, and further containing yttriumtrifluoroacetate and barium trifluoroacetate, on a substrate, subjectingan oxide superconducting precursor, which is obtained by a heattreatment for calcination, to a heat treatment for crystallization, andforming a thick oxide superconductor film on said substrate.
 10. Themethod of producing the oxide superconductor in the form of a thick filmtape according to claim 9, wherein in said heat treatment forcalcination, a heating rate is 20° C./min or more.
 11. The method ofproducing the oxide superconductor in the form of a thick film tapeaccording to claim 9, wherein in said heat treatment for calcination,the product of the traveling speed of the substrate and the temperaturegradient is preferably 20° C./min or more.
 12. The method of producingthe oxide superconductor in the form of a thick film tape according toclaim 9, wherein said heat treatment for calcination is carried out at250° C. or higher in the atmosphere having a water vapor partialpressure of not more than 2.1% by volume.
 13. The method of producingthe oxide superconductor in the form of a thick film tape according toclaim 9, wherein a difference between the maximum thickness portion andthe minimum thickness portion in said thick oxide superconducting filmis 1 μm or less.
 14. The method of producing the oxide superconductor inthe form of a thick film tape according to claim 9, wherein the amountof change in critical current density is ±0.5 MA/cm².
 15. The method ofproducing the oxide superconductor in the form of a thick film tapeaccording to claim 9, wherein said thick oxide superconducting film iscomprised of RE_(1+x)Ba_(2−x)Cu₃O_(y)(wherein RE represents at least oneelement selected from the group consisting of Y, Nd, Sm, Gd, Eu, Yb, Prand Ho; x represents the number of 0≦x≦0.4; and y represents the numberof 6.5≦y≦7.0).
 16. A method of producing an oxide superconductor in theform of a thick film tape comprising the steps of: coating a compositionfor a thick oxide superconducting film, containing a copper salt of abranched saturated aliphatic carboxylic acid having 6 or more carbonatoms or a copper salt of an alicyclic carboxylic acid having 6 or morecarbon atoms, and containing an yttrium salt of a branched saturatedaliphatic carboxylic acid having 6 or more carbon atoms or an yttriumsalt of an alicyclic carboxylic acid having 6 or more carbon atoms, andfurther containing barium trifluoroacetate, on a substrate, subjectingan oxide superconducting precursor, which is obtained by a heattreatment for calcination, to a heat treatment for crystallization, andforming a thick oxide superconductor film on said substrate.
 17. Themethod of producing the oxide superconductor in the form of a thick filmtape according to claim 16, wherein in said heat treatment forcalcination, a heating rate is 2° C./min or more.
 18. The method ofproducing the oxide superconductor in the form of a thick film tapeaccording to claim 16, wherein in said heat treatment for calcination,the product of the traveling speed of the substrate and the temperaturegradient is preferably 2° C./min or more.
 19. The method of producingthe oxide superconductor in the form of a thick film tape according toclaim 16, wherein said heat treatment for calcination is carried out at250° C. or higher in the atmosphere having a water vapor partialpressure of not more than 2.1% by volume.
 20. The method of producingthe oxide superconductor in the form of a thick film tape according toclaim 16, wherein a difference between the maximum thickness portion andthe minimum thickness portion in said thick oxide superconducting filmis 1 μm or less.
 21. The method of producing the oxide superconductor inthe form of a thick film tape according to claim 16, wherein the amountof change in critical current density is ±0.5 MA/cm².
 22. The method ofproducing the oxide superconductor in the form of a thick film tapeaccording to claim 16, wherein said thick oxide superconducting film iscomprised of RE_(1+x)Ba_(2−x)Cu₃O_(y)(wherein RE represents at least oneelement selected from the group consisting of Y, Nd, Sm, Gd, Eu, Yb, Prand Ho; x represents the number of 0≦x≦0.4; and y represents the numberof 6.5≦y≦7.0).